The present invention relates to the art of twisting ball electric-paper displays. It finds particular application in conjunction with fabrication of polarized small balls for use in an electric-paper display sheet.
An electric paper display device, also known as gyricon-based electric paper display, is typically in sheet form as described in U.S. Pat. Nos. 4,126,854 and 4,143,103 assigned to the present assignee and which are fully incorporated herein by reference. The electric paper display comprises a thin transparent sheet having many of the attributes of paper documents. It looks like paper, has ambient light valve behavior like paper (i.e. the brighter the ambient light, the more easily it may be seen), flexible like paper, can be carried around like paper, can be written on like paper, can be copied like paper, and has nearly the archival memory paper. It is also possible to form the display device as a rigid structure incorporating an array of addressing electrodes. In both embodiments, the salient features are an elastomeric host layer a few mils thick which is heavily loaded with bichromal balls tens of microns in diameter. Each bichromal ball has hemispheres of contrasting colors, such as a white half and a black half, and is contained in its own spherical cavity filled with a dielectric liquid. Upon application of an electrical field between electrodes located on opposite surfaces of the host layer, the balls will rotate according to the interaction of their dipole and depending on the polarity of the electric field generated, presenting one or the other hemisphere to an observer.
In a previous method for fabricating bichromal balls as shown in U.S. Pat. No. 5,344,594 assigned to the present assignee, monochromatic glass balls are formed and heavily loaded with titanium dioxide so as to appear white. The balls are deposited in a monolayer upon a substrate. Then the surface of each ball is coated from one direction in a vacuum evaporation chamber with a dense layer of nonconductive black material which coats one hemisphere. The bichromal balls are mounted in liquid filled cavities and suspended in a host matrix. Both the liquid surrounding the balls and the balls themselves are dielectric. Therefore, although the balls are macroscopically electrically neutral, on a microscopic scale they have an electrical double layer comprising two layers of charges of opposite sign. One charge layer is localized at the surface of the ball and the other charge layer is in the nature of a space charge extending outward from the surface of the ball into the dielectric liquid.
A measurable aspect of the electrical double layer, known as the zeta potential, is the net surface and volume charge that lies within a shear surface associated with the motion of the ball through the dielectric liquid. For a given liquid, the zeta potential is a function only of the ball surface material. Thus, the material properties which give rise to differences associated with the color or reflectivity of each hemisphere of a ball gives rise to different characteristic zeta potentials with respect to the dielectric liquid in the cavity containing a ball. It is the difference in zeta potential between the hemispheres of the ball which causes the ball to act like a dipole in the presence of an electrical field generated by oppositely disposed electrodes. In other words, each ball will rotate until its dipole moment lines up with the direction of the electrical field established between the opposed electrodes.
In addition to the dipole charge distribution found on the bichromal ball in the presence of an electrical field, there is also a monopole charge with the net electrical charge. It is quite unlikely that the two hemispheres of a ball having zeta potentials of opposite polarity will have the same magnitude. However, if that is the case, a monopole charge will not be established. As a result of the monopole charge, a ball is caused to translate in the direction of the electrical field and will rest and be retained against the cavity wall. In order for the ball to rotate easily in the liquid within the cavity, due to the dipole charge, it must move from contact with the cavity wall. When at rest against the cavity wall, friction and other forces will prevent it from rotating until it has been moved away once again, due to the monopole charge. It is this feature which enables long term image retention in this display device.
Other methods of fabricating bichromal balls are described in U.S. Pat. Nos. 5,262,098 and 5,344,594 assigned to the present assignee. In the '098 patent, a spinning disc method is described. In the '594 patent, balls are formed by the flowing together of two side-by-side streams of differently colored hardenable liquids into the center of a laminarly flowing host liquid. As the bichromal stream is transported by the host liquid as a free jet, its forward end becomes unstable and breaks up into droplets which form into spherical balls as they are moved by the host liquid. Further transport of the balls by the host liquid moves them past a curing station and a separating station.
The prior art methods of fabricating bichromal balls for an electric paper display involve a chemical process which adds a chemical charge to the surface of each ball by applying differently charged dyes or pigments to the ball hemispheres. The hemispherically-differential surface chemistry, thus, creates a dipolar ball which will align in an external electric field. Each half of a ball is made with differentiating pigment and/or constituent materials, which charge to different degrees in the fluid in which the balls rotate. Alternately, a hemispherical coating is applied to one hemisphere of a monochromal ball to differentiate its surface charge characteristics from the uncoated hemisphere. The chemical surface charging of bichromal balls suffers from being susceptible to neutralization by triboelectricity or the presence of impurity ions adjacent to and/or mixed within the charged surface coating.
The present invention contemplates a new and improved method of fabricating twisting balls for electric-paper displays which overcomes the above-referenced problems and others.